The present invention relates to a method and apparatus for treating a fiber suspension, in which the method fiber suspension is fed to the treatment apparatus, and impurities and too large particles are separated from the suspension. The method and the apparatus in accordance with the present invention are especially suitable for screening suspensions in the sorters of the pulp and paper industry.
The sorters known from the prior art most often comprise an apparatus to which pulp is fed continuously and which has one or more stationary screen cylinders and a rotor moving adjacent to the surface of these cylinders. Pulp is fed axially to the space between the screen surfaces/surface and the rotor, subsequent to which and due to the operation of the rotor the pulp is brought into a circulating, spiral movement. During the screening the acceptable fraction passes through the screen surface and is discharged as an accept and the coarser fraction flows in axial direction along the screen surface from the inlet end to the outlet end of the cylinder, from which end it is discharged as a reject.
FIG. 1 is a schematic illustration of the above described prior art sorter, which has for simplicity only one screen cylinder and a closed rotor inside the cylinder. FIG. 2 is a graphic illustration of how by means of showing the described method the amount of the reject is increased between the screen surface and the rotor evenly as a function of the height of the cylinder. FIG. 3 is a corresponding graphic illustration of the amount of the accept flowing through the screen cylinder as a function of the height of the screen cylinder. It can be seen that the accept flow is at its maximum, of course, at the upper end of the screen cylinder, and therefrom the amount of accept decreases rapidly, because the relative portion of the reject in the material to be screened increases rapidly and on the other hand the reject tends to clog the screen surface. Thus the problem is that the actual capacity of the screen surface is used only partially, in other words the screen surface is used only at partial efficiency.
A possible solution to the above described problem could be, e.g. the use of a conic rotor, whereby more material to be screened may be introduced between the screen cylinder and the conic rotor. But also, in this case, the relative portion of the reject in the space between the screen surface and the rotor increases in the similar way as in a previously described arrangement. However, it has to be admitted that a slightly better screening capacity of the screen surface is achieved by a conic rotor than by a rotor in accordance with FIG. 1, because fresh unsorted suspension may be fed deeper in the space between the screen surface and the rotor.
A second possible solution is described in U.S. Pat. No. 4,642,189 which discloses a substantially conic rotor, the upper end of which is, however, formed in a way that the pulp to be fed is divided into three annular portions one within the other, is fed in at a different height in the space between the rotor and the screen surface. FIGS. 4 and 5 are graphic illustrations showing the amount of the reject and of the accept flow passing through the screen surface as a function of the height of the screen cylinder. It can be seen in FIG. 4 that the amount of the reject increases gradually according to the different feed points of the material. A broken line in the figure shows the situation, in which the feed between the rotor and the screen surface is carried out in one stage. Thus the slashed area shows the advantage achieved with the arrangement in accordance with said publication compared with the arrangements described above. FIG. 5 clearly illustrates the three-stage-nature of the feed of the material. Each feed stage generates an addition, in other words a step, to the accept flow, and after each feed the accept flow reduces according to FIG. 3, yet in a way that the speed, of reducing of the accept flow after each feed increases the deeper the feed between the rotor and the screen surface is carried out, because the increasing amount of reject decelerates the accept flow more intensively the greater the amount of the reject in the space is. The broken line in the figure signifies the situation in which the feed is carried out in one single stage directly from the end of the screen cylinder. The slashed area shows the amount of the volume flow which additionally passes through the screen surface by using this method of feeding.
In the patent, the feed stages of the unsorted pulp take place in the steps of about 20% considering the total height of the screen cylinder. It is not advantageous to arrange feed stages below the vertical middle of the screen cylinder, because they would hardly be of any use anymore compared with the above described curves. Since it has been necessary to arrange a feed of dilution liquid through the rotor to the lower part of the screen cylinder, the problem with the arrangement in accordance with said invention seems to be the excessive thickening in the space between the rotor and the screen surface.
A common problem to all described arrangements of the prior art is the even increase of the amount of the reject between the screen surface and the rotor. Because the reject generates a particular flow resistance to the screening process, the screening efficiency achieved is not much better than that of a traditional screen apparatus illustrated in FIG. 1, unless it is possible to remove reject from the area between the ends of the screen apparatus, in other words to reduce the flow resistance in the space between the rotor and the screen surface caused by the reject.